Separation of metals



Sept. 15, 1936. H. N. GILBERT SEPARATION OF METALS Filed Au 29, 1935 u Mr;

INVENTOR. Harvey M GI/ber'f A TTORNEY.

Patented Sept. 15, 1936 UNITED STATES SEPARATION OF METALS Harvey N. Gilbert, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware Application August 29, 1933, Serial No. 687,384

22 Claims.-

This invention relates to the purification of alkali metals, and more particularly to the removal of alkaline earth metals therefrom.

Alkali metals made by fused salt electrolysis processes often contain small amounts of alkaline earth metals as impurities due to the occurrence of alkaline earth metal salts with thealkali metal salts in the electrolytes. For example, in the preparation of sodium by the fused salt electrolysis of sodium chloride, a suitable electrolyte contains calcium chloride, and the crude metal obtained from the electrolysis of this mixture contains appreciable amounts of calcium. At the elevated temperatures at which the electrolysis takes place, calcium is relatively soluble in sodium, but by cooling to a point close to the melting point of sodium, calcium may be precipitated out. This phenomenon may be utilized in purifying the crude sodium. 'I'hus the crude, molten sodium is cooled to a temperature near its melting point and filtered to substantially remove the precipitated calcium, together with any salts and oxides which may be present. However, this procedure does not completely remove the calcium; small amounts of calcium will remain in the filtered sodium, probably because the calcium is slightly soluble in sodium at the temperature of filtration.

An object of this invention is to provide a meth ed for substantially completely removing alkaline earth metals from alkali metals. A further object is to substantially completely remove calcium from sodium. Other objects will be hereinafter apparent.

These objects are attained by subjecting molten alkali metal containing small amounts of alkaline earth metal to the action of an oxidizing agent, whereby the alkaline earth metal is oxidized and then removing the precipitated oxide by filtration or other mechanical means. oxidizing agents suitable for accomplishing the objects of compounds or mixtures, e. g. water vapor or air.-

I have found that in order to effect oxidation of the alkaline earth metals in accordance with my invention, the oxidizing agents must be contacted with the mixture of alkali metal and alkaline earth metal as herein described at a temperature in the neighborhood of .350 C. or higher. At temperatures below about 350 C., little or no oxidation occurs; at about 350 C., subs Such peratures between 400 and 600 0.; there is little advantage in operating at or above the boiling 5 point of the alkali metal, since atthose temperatures separation would be effected by distillation. An advantage of my process is that it provides a means of removing the alkaline earth metal without resorting to distillation tempera- 1o tures.

In one method of carrying out my invention, I utilize alkali metal hydroxide as the oxidizing agent. For example, to remove calcium from sodium I may contact the sodium with a bath of molten sodium hydroxide at a temperature of 400 to 600 C., whereupon the calcium reacts with the sodium hydroxide to produce calcium oxide and set free sodium. After the reaction is complete, the sodium is separated from the molten caustic by gravity, leaving the precipitated calcium oxide in the sodium hydroxide. If desired, the purified sodium then may be filtered at a temperature slightly above its melting point in order to remove any caustic which may .have escaped separation. Preferably, I contact the molten sodium with the molten caustic by allowing a small stream or droplets of the molten metal to rise through a body of the caustic bath. This is advantageously done by allowing the sodium to rise through a long, narrow, substantially vertical tube open at both ends and immersed in a bath of fused sodium hydroxide. The purified sodium separates as a layer floating on the surface of the sodium hydroxide molten bath, while the calcium oxide formed settles to the bottom of the caustic bath. During the operation, the purified alkali metal should be protected from oxidation by the air; this may be accomplished by carrying out the process under an atmosphere of nitrogen or othersuitable inert gas. From time to time, the

accumulated calcium oxide may be removed by ladling it from the bottom of the caustic container and/or by filtering the molten caustic.

An'altemativemethod of practicing my inven- 5 tion comprises stirring the impure molten alkali metal at a temperature of about 350 C. or higher with suflicient alkali metal hydroxide to oxidize the alkaline earth metal present. Preferably, I use an excess of the hydroxide.

- In carrying out my process, it is desirable that.

the alkali metal hydroxide be substantially anhydrous in order toprevent undue loss of alkali metal by reaction with water and to prevent exqessive generation of hydrogen, especially when is advantageously dehydrated by a preliminary passage of alkali metal therethrough. After the alkali metal has been passed through the bath for a period of time which will depend upon the amount of uncombined water present, the bath will be completely dehydrated and thereafter sub-- stantially no loss of alkali metal by oxidation will occur.

Molten caustic alkalis are known'to have a certain solvent eifect on alkali metals. For example, molten sodium hydroxide or potassium hydroxide will dissolve sodium or potassium. The amount of metal dissolved will vary with the temperature, the solubility of the metal in the molten caustic decreasing as the temperature is raised. For example, at around 500, 100 parts by weight of sodium hydroxide will dissolve 20 to 25 parts by weight of sodium, while at around 700 C. the same quantity of hydroxide dissolves 8 to 10 parts of sodium. Hence, when alkali metal is introduced into molten caustic in accordance with my invention, a certain amount of the alkali metal will be dissolved by the caustic until a saturated solution is formed; thereafter substantially all of the alkali metal introduced will pass through the caustic bath and may be recovered without loss. I am aware that it has been known heretofore that calcium will react with molten sodium hydroxide to produce sodium and hydrogen. This reaction occurs very violently, usually with detonation. However, I have discovered that when sodium containing small amounts of calcium, for example, upto around 10% by weight, is contacted with molten sodium hydroxide, the reaction between the caustic and the calcium occurs quietly without detonation and produces little or not appreciable rise in temperature.

Anapparatus suitable for carrying out one of the modifications of my invention is illustrated in the appended drawing. In the drawing, I is a feed tank for impure alkali metal; 2 is a covered charging opening for introducing said metal; 3 is a connection to a source of inert gas; 4 is an outlet pipe extending downward through a bath of purifying agent (e. g. molten sodium hydroxide) contained in vessel 6. The lower end of pipe 4 is directed upward and terminates in a construction or nozzle 1. .A valve 5 controls the flow of metal through pipe 4. 8 represents a coil of pipe, the lower open end of which extends down-ward and is positioned directly over nozzle 1. The upper end of coil 8 terminates in a gravity separating chamber 9, the lower end of which is open and the upper end of which leads, through connections I0 and I2 to a receiver for the purified metal, I3. I I is a connection to a source of inert gas; It is an outlet connection provided with a valve l5. Vessels I, 6 and I3 may be heated by any suitable means (not shown on the drawing) In the operation of the above described apparatus in accordance with one method of practicing my invention, tank 6 is filled with a suitable liquid oxiding agent, e. g., molten sodium hydroxide and the temperature thereof is maintained at 400 to 800C. by heating means not shown. The sodium to be purified is placed in tank i, where it is melted by suitable heating means not shown. The valve molten sodium hydroxide. The calcium oxide, most of which is swept over into separator 9 by the flow of sodium through coiled pipe 8, settles to the bottom of tank 6 by way of the lower part of separator 9. The purified sodium rises into the upper part of the separator 9 and from thence flows through pipe Hi to the receiver 13. The height of pipe l0 above the layer of the melt in tank t is so adjusted that the sodium readily siphons over through pipe ill, by reason of the difference between the respective specific gravities'of the molten sodium and the molten sodium hydroxide.

My invention is further illustrated by the following example:

Example A quantity of sodium hydroxide was melted in an iron pot and brought to a temperature of 450" C. Sodium was added to the molten caustic in small quantities until the bath was substantially completely dehydrated. A vertical coil of iron pipe then was placed in the molten caustic with the lower end of the pipe open to the caustic bath. The upper end of the coil was connected to a separating vessel, which was partly submerged in the molten caustic. An opening was provided in the bottom of the separating vessel, and an outlet pipe at the upper end of the separating vessel was connected to a closed receiver. The apparatus was arranged to maintain an atmosphere of inert gas above the liquid in the separating vessel and receiver. Sodium containing a small amount of calcium was melted and introduced into the lower end of the iron coil at a constant rate so as to form a train of sodium globules rising through the caustic in the. iron coil. The sodium and caustic passing from the upper end of the coil into the separating vessel, separated into two liquid layers, the sodium floating on the caustic. As the process proceeded, the purified sodium flowed from the upper part of the separating vessel into the receiver, while the caustic which separated out flowed through the bottom outlet of the separating vessel into the main caustic bath.

Before the treatment with caustic, an analysis showed the sodium to contain 0.045% of calcium; no trace of calcium could be found in the treated sodium which collected in the receiver. The purified sodium also was substantially free from oxides and caustic.

Another method of carrying out my invention comprises contacting the alkali metal with an alkali metal oxide at a temperature of 350 C. or higher. For example, sodium may be treated to remove calcium by agitating the molten metal with a small amount of sodium oxide at 400 to 600 C. and then removing excess sodium oxide and precipitated calcium oxide by filtration. Equivalent results may be obtained by contacting the molten metal with a gaseous oxidizing agent, e. g., air or water vapor, at a temperature of about 350 C. or higher until the alkaline earth metal present is completely oxidized. Possibly, in this case, part of the alkali metal is converted to the oxide or hydroxide, which in turn reacts with the alkaline earth metal. If desired, the oxidizing gas may be diluted with an inert gas such as nitrogen or hydrogen. After the oxidation treatment, the treated metal may be filtered to remove the precipitated oxide. Thus to purify sodium containing a smallamount of calcium, the molten "metal is heated to 350 C. or higher, and air, water vapor or a mixture thereof is passed through the molten metal until a filtered sample is found to be free from calcium. The molten metal is then filtered by known means (as, for example, by means'of the method and apparatus described in my U. S. Patent 1,943,307) to recover the pure sodium.

,The amount of air and/or water vapor used is hydroxide, the reaction of the alkaline earth metal with the hydroxide would result in the formation of a small amount of potassium, which would alloy with the sodium. Hence, when it-is desired to avoid such alloy formation, I prefer to use an oxide or hydroxide of the metal being treated.

My novel process may be applied to alkali metal which has been partially freed from alkaline earth metal by filtration at a temperature at which the solubility of the alkaline earth metal is low or the method may be applied'directly to crude alkali metal as obtained from electrolytic cells. For example, sodium obtained by electrolyzing a fused mixture of sodium chloride'and calcium chloride ordinarily will contain appreciable quantities of calcium. This crude sodium may be treated according to my invention, preferably by air blowing and subsequent filtration,

to obtain a pure product substantially free from oxides, salts and calcium. x

I claim: 1. A process for the purification of an alkali metal containing small amounts of alkaline earth imetal comprising subjecting said alkali metal in the molten state to the action of an oxidizing agent at a temperature not lower than-about 350 C. to convert said alkaline earth metal to its oxide and separating purified metal from the 7 metal containing small amounts of alkaline earth oxide.

2. A process for the purification of an alkali metal comprising subjecting said alkali metal in the molten state to the action of an alkali metal hydroxide at a temperature of 350 to 600 C. to convert said alkaline earth metal to its oxide and separating purified metal from the oxide.

3. A process for the purification of sodium containing small amounts of calcium comprising subjecting said sodium in the molten state to the action of an oxidizing agent at a temperature not lower than about 350 C. to convert said calcium to its oxide and thereafter filtering, to remove oxide.

4. A process for the purification 01 sodium containing small amounts of calcium comprising subjecting said sodium in the molten state to the actioniof an oxidizing gas at a temperature not lower than about 350 C. to convert said calcium to its oxide and thereafter filtering toseparate putt-. 'fied metal from the oxide.

to its oxide and separating purified metal from the oxide.

6. A process for the purification of sodium containing small amounts of calcium comprising subjecting said sodium in the molten state to the l action of sodium hydroxide to convert said calcium to its oxide and separating purified metal from the oxide.

'7. A process for the purification of sodium containing small amounts of calcium comprising subjecting said sodium in the molten state to the action of sodium oxide to convert said calcium to its oxide and separating purified metal from the oxide.

8. A process for the purification of an alkali 16 metal containing small amounts of calcium comprising blowing air through said alkali metal. at

a temperature of 400 to 600 C. and thereafter filtering to remove oxide.

9. A process for the purification of sodium coni taining small amounts of calcium comprising blowing a gaseous oxidizing agent through said sodium at a temperature of 350 to 600 C. and thereafter separating oxide from the metal. 10. A process for the purification of sodium containing small amounts of calcium comprising blowing air through said sodium at a temperature of 400 to 600 C. and thereafter filtering to remove oxide.

11. A process for the purification of an alkali metal containing small amounts of alkaline earth metal comprising flowing said alkali metal in the molten state through a body of molten alkali metal hydroxidemaintained at a temperature of 350 to 600 C. and separating purified metal from said molten hydroxide.

12. A process for the purification of an alkali metal containing small amounts of calcium comprising flowing said alkali metal in the molten state through a body of molten caustic alkali maintained at a temperature of 350 to 600 C. and

separating purified metal from said caustic alkali.

13. A process for the purification of sodium containing smallamounts or alkaline earth metal comprising flowing said sodium in the molten state through a body of molten alkali metal hydroxide maintained at a temperature of 350 to 600 C. and separating purified metal from said caustic alkali by gravity separation.

14. A process for the purification of sodium containing small amounts of calcium comprising flowing said sodium in the molten state through a body of molten sodium hydroxide at a temperature of 400 to 600 C. and separating purified metal from said caustic alkali by gravity separation.

15. A process for the purification of an alkali metal containing a small amount of an alkaline earth metal comprising subjecting said alkali metal in the molten state at a temperature of not less than about 350 C., to the action of a substance containing oxygen which is capable of oxidizing said alkaline earth metal and separating alkaline earth metal oxide from the purified molten alkali metal.

16. A process for the purification of sodiuni containing a small amount of calcium comprising subjecting the impure sodium in the molten state A to the action of an oxygen-containing gas at atemperature of 400 to 600 C., and thereafter" 1 filtering the molten sodium to remove oxide there-,

metal in the molten state, at a temperature not lower than about 350 C., to the action of an oxidizing agent capable of converting said alkaline earth metal to its oxide, while controlling the oxidizing action in such manner that the extent of oxidation is not greatly in excess of that required to oxidize all of the alkaline earth metal present and finally removing oxide from the purifled molten alkali metal.

18. A process for the purification of an alkali metal containing a small amount of an alkaline earth metal which comprises subjecting said a1- kali metal in the molten state, at a temperature not lower than about 350 C., to the action of an oxidizing agent, while restricting the extent of oxidation in such manner that no more than a small fraction of the alkali metal is oxidized when the alkaline earth metal present is substantially completely oxidized and finally removing oxide from the purified molten alkali metal.

19. A process for the purification of an alkali metal containing a small amount of an alkaline earth metal which comprises oxidizing said metal in the molten state at a temperature not lower than about 35 C., to form an amount 01' alkali metal oxide sufllcient to react with said alkaline earth metal and convert the latter to its oxide. 5

20. A process for the purification of an alkali metal containing small amounts of alkaline earth metal comprising subjecting said metal in the molten state at a temperature of not lower than about 350 C. to the action of alkali metal oxide 10 to convert said alkaline earth metal to its oxide and separating purified alkali metal from the oxide.

21. The process according to claim 20 in which the alkali metal oxide is. formed within the body 15 of alkali metal by the oxidation of a small part of said alkali metal.

22. A process according to claim 7, in which the sodium oxide is formed within the body of said sodium by oxidation of a small part of said 2 sodium.

HARVEY N. GILBERT. 

